Any serious hobbyist needs a decent bench top power supply to work with. Specifically I required a power supply to use for testing audio amplifiers. The supply needed to be a dual supply and capable of delivering a few Amps current for amplifier testing and have some means of protecting itself and the circuit in question from overload or short circuit.

Specification

+/-30V Dual variable regulated Supply.

3A Maximum output current with overload trip.

Description

This was my first serious electronics project and I began working on this project in my second to last year of school at age 16. I was attending Pinetown Boys High School in South Africa (www.pbhs.co.za) Since I had limited resources and funds available I had to find ways of implementing it cheaply, for example by winding the transformer myself, but I still intended to do a decent job and wanted it to be adequate for my needs for a long time (I still use this supply today and it continues to function well).

I opted to use the well known LM317/337 regulator pair for the positive and negative rails. In order to implement the protection circuit I opted for a “trip and reset” circuit as opposed to a current limit. This I implemented using a discrete flip-flop circuit using a series current sense resistor to trigger a change in state of the flip-flop. The output of the flip-flop controlled a simple transistor/zener diode series regulator circuit that is positioned ahead of the LM317/337 regulators. In the event of the load current exceeding the pre-set maximum, the flip-flop would 'trip' thus cutting off the supply to the zener diode series regulator and hence the LM317 or LM337 regulator. The flip-flops are reset using a push switch on the front of the supply and an LED indicates that the supply has tripped. Each rail has its own separate trip and reset. Although perhaps somewhat crude this overload protection has saved the supply and many a circuit from destruction over the years.

I wound the transformer by pulling apart an old transformer I was given and then rewinding it to my own specification. I required sufficient voltage headroom to drive the series regulators.

I designed the printed circuit boards on a drawing board using pencil and paper (and lots of eraser). Once happy with the routing I went over the track with ink and then photo-copied it onto transparency. I then used Kontak Chemi's Positiv 20 (www.kontaktchemie.dk) photosensitive spray to create the etch resist layer and etched the boards using ferric Chloride (pretty standard home brew stuff).

For the box I was fortunate to know someone who worked for an electronics company who got a colleague to bend a box out of aluminium for me to my specification and then I had to drill and file out all the holes. I spray painted it and did all soldering and wiring myself.

Internal view of the power supply

The supply was complete before I entered my final year of school and I was encouraged to enter it in the inter-schools “Young Scientist Expo” held at the University of Natal in Durban, South Africa. I was awarded a Second Class award for the project, due to lack of “originality” (there is nothing particularly special about a bench top power supply), however my original intention had not been to win any awards for it, but to use it for my own hobby work.